This invention relates to control systems for externally controlled converters in general and more particularly to an improved control system which has less sensitivity to variations in line frequency, harmonics in the line voltage and commutation interference.
Various control systems for such externally controlled converters have been previously developed for example see German Auslegeschrift No. 1,021,025 or a handbook by G. Moeltgen entitled "Line Controlled Converters with Thyristors", edited and published by Siemens AG 1967 pages 275 and 280. These previous systems have triggered on the zero crossing of a synchronization voltage which is typically the line voltage. This voltage however can sometime shift several degrees if harmonics are present. Sometimes because of harmonics or other interferences, a plurality of zero crossings can occur successively. Such can occur, for example, if other converters are coupled to the same line when their respective commutation occurs. Thus, under certain conditions the prior art control systems do not perform in a satisfactory manner, particularly when they are used for controlling thyristors in positioning control circuits and are connected to a synchronizing voltage which has a variable frequency and heavy distortion of the line voltage wave shape. In attempts to overcome these problems filters made up of resistors and capacitors or inductors and capacitors have been used. However these filters are considerably expensive and do not work in a completely adequate manner. In particular such a solution is not effective where the frequency of the synchronization voltage changes. Thus there is a need for an improved control system for externally controlled converters which is less sensitive to the above described effects of frequency variation, harmonics and interference from other converters.